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Wednesday, September 24, 2008

Magnetar flashes astronomers

Magnetars are the cores of massive stars that blew up in supernova explosions.NASA/JPL-Caltech

Astronomers believe they have spotted an aurora around of one of the densest objects in the Universe.

In the first observation of its kind, researchers say that they have seen optical light from a magnetar, an extremely dense, dying neutron star with a very powerful magnetic field. Astronomers watched for days as the magnetar flared, brightening at one point by over 200 times in just four seconds. The flash was far too bright to come from any normal stellar processes, and the leading explanation is that the light was emitted from ions accelerated by its magnetic field.

"It's a little bit similar to what you'd see in the Northern Lights," says Alexander Stefanescu, an astronomer Max-Planck Institute for Extraterrestrial Physics in Garching, Germany, and an author on one of two papers about the flares in this week's Nature1,2.

The mysterious object was first spotted as a gamma-ray burst by the orbiting Swift spacecraft on 10 June 2007. Within minutes, optical telescopes around the world swivelled to observe the source of the activity. Much to their surprise, they saw a series of bright optical flashes. The first occurred just minutes after the initial burst, but the source continued to flare for a few nights.

The observations showed that whatever the object was, it was in our own Milky Way and no more than 10,000 to 16,000 light years away, less than a quarter of the distance across the Galaxy. Moreover, the energy contained in the flares was far too great to have come from a normal star, Stefanescu says.

In a spin

The suspected source of the flashes is an exotic variety of neutron star — neutron-rich remnants of stars that have exploded in a supernova. Theorists believe that neutron stars contain a smattering of charged particles that create its magnetic field, although how they do so is still not entirely understood.

But in rare cases, some neutron stars can develop an even stronger field. These magnetars can spin at rates of many revolutions per second, giving them a magnetic field up to 1,000 times that of a regular neutron star. "This field is actually so strong that it could erase a credit card from the distance of the moon," Stefanescu says. "It could rip your flesh off you from 1,000 kilometres."

Only 15 magnetars have ever been seen, according to Stefanescu. Most are invisible at optical wavelengths, but can occasionally be seen as bursts of high-energy gamma-rays and x-rays, possibly triggered by massive starquakes in the magnetar's crust. But such surface shifts would be too powerful to create the lower-energy optical flashes.

Stefanescu says that the most likely sources of the flashes are ions and electrons being accelerated through the turbulent magnetic fields above the magnetar's surface. As the charged particles spiral along the magnetar's powerful field lines, they emit radiation in a way that's somewhat analogous to the way the same particles create Earth's own celestial light shows, the aurora borealis and aurora australis. But he admits that "the exact details of how it works are not really well understood".

It's an intriguing find, says Chryssa Kouveliotou, an astronomer at the NASA Marshall Space Flight Center in Huntsville, Alabama. Kouveliotou finds the burst so odd that she remains "on the fence" over whether the source really is a magnetar. "I'd like to see at least one more example before I put my money down," she says.